IDing the Sigma 17-70 f2.8-4 (s)

Hello. I am looking to broaden my horizons past my 18-200 VR lens. In my research, I've been looking into the Sigma 17-70 f2/8-4. But there seem to be two different lenses called the same thing. In the lens comparison table, they look different and have some elements that are different, as well. Am I right about that? How can you tell what lens is which? Are both for sale? Has one replaced the other? Can somebody clear up what I'm sure is really a very simple question, but one which has me stymied for the moment? Thanks very much.